The present invention relates generally to class H amplifiers, and more particularly to improvements which improve their efficiency, reduce coupling of power supply signals into the amplifier outputs thereof, and increase the speed of operation while maintaining relatively low quiescent power consumption therein. The invention relates yet more particularly to improving the efficiency and performance of xDSL line driver circuits that include class H amplifiers. The term “xDSL” represents several variations of DSL (Digital Subscriber Line), including ADSL (Asymmetric Digital Subscriber Line), HDSL (High-bit-rate Digital Subscriber Line), VDSL (Very High-bit-rate Digital Subscriber Line), and other variations.
Prior Art FIG. 1 shows the circuitry for the positive supply of a conventional class G amplifier. Class G amplifiers are those which require multiple external power supplies. In FIG. 1, the high side supply voltage terminal 2 of an amplifier 1 is connected to the cathode of a switching diode D. The anode of diode D is connected to a relatively low magnitude first supply voltage VCCL. Supply voltage terminal 2 of amplifier 1 also is connected to the emitter of an NPN pass transistor 3, the collector of which is connected to a relatively high magnitude second supply voltage VCCH. The first supply voltage VCCL typically is chosen to have a value that most of the time is close to the average value of the amplifier output signal V. The second supply voltage VCCH is switched onto amplifier supply voltage terminal 2 whenever it is necessary to process a peak value of the amplifier signal voltage VIN, which for example may be a DMT (discrete multi-tone) signal as illustrated in FIG. 3. A main shortcoming of conventional class G amplifiers is the doubling of the number of required power supplies and the associated total cost.
The graph of FIG. 3 illustrates various large-amplitude, short-duration signal peaks P that typically occur in a DMT signal. The illustrated signal peaks P are random and occur between low level adjacent portions of the signal to be amplified.
Prior Art FIG. 2 shows the circuitry for the positive supply of a conventional class H amplifier that includes switching diode D with its cathode connected to the high side supply voltage terminal 2 of amplifier 1. Terminal 2 also is connected to one plate of a capacitor C, which is sometimes referred to as a “pump capacitor” because it is used to pump up the supply voltage. The other plate of capacitor C is connected to the emitter of pass transistor 3 and to one terminal of a current source I having its other terminal connected to a low side supply voltage such as ground or VEE. The collector of pass transistor 3 and the anode of switching diode D are connected to a single supply voltage VCC.
Class H amplifiers are similar to class G amplifiers in that class H amplifiers also operate from a lower supply voltage than conventional Class AB amplifiers in order to achieve the same output voltage swing as conventional class AB amplifiers. But rather than requiring a separate high voltage supply, class H amplifiers use a charged capacitor C to supply the energy needed to process the signal peaks P (FIG. 3) received by driver amplifier 1. This is practical because the above mentioned signal peaks P are generally relatively infrequent and of short duration, thereby allowing adequate time for charging capacitor C between the signal peaks P.
A shortcoming of conventional class H amplifiers, especially when used to generate supply voltages to xDSL driver amplifiers, is that if pass transistor 3 is driven with a fast square wave, the class H amplifier raises and lowers the driver amplifier supply voltage (e.g., on high side terminal 2) too fast. It is well-known that the CMRR (common mode rejection ratio) and PSRR (power supply rejection ratio) of an amplifier deteriorate as the signal frequency is increased, and it can be shown that the output signal of an amplifier may be distorted if its supply voltages are raised or lowered too quickly. Because of the finite power supply rejection capability of the xDSL driver amplifier 1, a fast dV/dt signal on its power supply rail(s) can be coupled into the amplifier output and cause distortion in the output signal. Furthermore, too much time is required for the current source I in FIG. 2 to recharge capacitor C if it is substantially discharged. There is a difficult trade-off between the magnitude of the current source I and the amount of time required to recharge the capacitor C because increasing the magnitude of current source I increases the power consumption. A conventional class H xDSL line driver typically has both its upper and lower supply voltages generated by upper and lower class H amplifiers, respectively, of the general type shown in FIG. 2.
Line drivers targeted at the xDSL market typically have employed a class AB output stage to achieve the power and linearity requirements of these applications. If the quiescent power of the line driver is assumed to be zero and the output signal VO can be driven all the way to the high side and low side supply rails, then the class AB output stage is about 78% efficient when driving a sinusoidal output signal into a load. The efficiency is given by the following equation:
                              η          AB                =                                            π              ⁢                                                          ⁢                              V                O                                                    4              ⁢                              V                CC                                              ×          100          ⁢                      %            .                                              Equation        ⁢                                  ⁢                  (          1          )                    
It can be seen from Equation (1) that as VO becomes less than the supply voltage VCC, the efficiency ηAB progressively decreases. Equation (1) can be used to calculate the class AB output stage efficiency ηAB as it pertains to a typical DMT signal. The VDSL2 down stream “Profile 8b” is an amalgam of quadrature amplitude modulated (QAM) tones which are spaced 4.3125 kHz apart. The tones are uncorrelated and there are a large number of them, so the amplitude distribution can be approximated as being Gaussian. The DMT signal is comprised of many individual tones (not shown in FIG. 3), and when these tones randomly become aligned in phase, a signal peak P occurs as shown in FIG. 3. The combination of all such aligned tones P produces a high peak-to-average ratio (PAR) signal. For example, a typical PAR for “Profile 8b” has a value of 5.5. (Profile 8b is one of the parameter specifications defined in the Telecommunications Standardization Sector (ITU) G.993.2 for VDSL2. Profile 8b is the highest power and thus stands to benefit the most from class H power efficiency.)
Equation (1) may be used to approximate what happens to the efficiency ηAB of the class AB stage when it is used with a high PAR (peak-to-average ratio) signal. For example, if VO is equal to VCC/PAR, Equation (1) indicates an efficiency ηAB of only 14%. One known technique that may be considered to improve this situation is to decrease VCC such that it is closer to the average value of the signal being amplified and then switch to a higher supply when it is necessary to process a signal peak P. The main disadvantage of this technique is that the signal fidelity may be substantially degraded.
The operation of the class G and class H amplifiers shown in FIGS. 1 and 2, respectively, can be described as either “tracking” operation or “gated” operation. If the amplifier is tracking (which is the method described in subsequently mentioned U.S. Pat. No. 6,636,103), it must monitor the input signal VIN, and be able to raise the supply voltage magnitude high enough and quickly enough to prevent the amplifier output signal VOUT from being clipped. This becomes more and more difficult to implement as the frequency content of the signal being amplified increases, and is relatively unfeasible with frequencies much above 12 MHZ for amplifiers fabricated using state-of-the-art integrated circuit process technology.
Considering a gated topology used in conjunction with the amplifiers shown in FIGS. 1 and 2, the high side supply voltage is switched in when needed, for example, by using a look-ahead signal provided by an external processor or a chip-set that drives the line driver integrated circuit. Typically, the processor or chip-set can be “aware” of signal characteristics such as the above mentioned peaks, up to approximately 400 nanoseconds before they actually appear at the input of the driver amplifier. Therefore, the gated amplifier can be alerted to the presence of a signal peak well in advance of the instant at which it arrives at the input of the driver amplifier. This can greatly reduce the needed bandwidth required for a “tracking” approach, and hence can reduce the power required to temporarily switch a high supply voltage to the appropriate supply voltage terminal of the driver amplifier.
Although there are various advantages and disadvantages to using both class G and class H amplifier topologies, many users tend to prefer the class H topology because it does not require doubling of the number of power supplies (one for the high side driver amplifier supply voltage and one for the low side driver amplifier supply voltage).
Above-mentioned U.S. Pat. No. 6,636,103, entitled “Amplifier System with On-Demand Power Supply Boost” and issued Oct. 21, 2003 to Wurcer et al., discloses an amplifier circuit wherein the supply voltages are raised automatically rather than by a signal generated by the processor. This makes the amplifier circuit easier to use, but the circuitry required to implement the technique disclosed in the '103 patent requires considerably higher power than is acceptable in many applications in which reduced power consumption is more important than ease of use.
Thus, there is an unmet need for a class H amplifier having improved efficiency of operation.
There also is an unmet need for an improved xDSL line driver circuit of the kind including a class H amplifier and having improved efficiency of operation.
There also is an unmet need for an improved, lower cost xDSL line driver circuit of the kind including a class H amplifier and having improved efficiency of operation.
There also is an unmet need for an improved class H amplifier which minimizes the coupling of the varying power supply signals into the driver amplifier output.
There also is an unmet need for an improved xDSL line driver circuit of the kind including a class H amplifier and having improved efficiency of operation and capable of increased operating speed while maintaining relatively low quiescent power.